1. Field of the Invention
This invention relates to method and apparatus for measuring the clearance between large rolls rotating close to each other. Such rolls are commonly used in rolling, pressing, sizing, and calandering operations performed on various materials such as paper, plastics, and many others. During such operations it is often desirable to measure the nip clearance between the closely rotating rolls.
2. Prior Art
Many prior art rolling apparatus provides for adjustment of roll clearance, but fails to provide for its measurement. See, for example, U.S. Pat. Nos. 2,310,262 (Shields); 2,825,217 (Byrd); 3,246,822 (Skeen); 3,670,644 (Hoever et al); and 3,845,708 (Liebert). Prior-art apparatus that does provide for nip measurement does so by measuring the distance between roll centers or by measurement of frame members in contact with the rolls. See, for example, U.S. Pat. No. 3,561,359 (Cohen). However, thermal expansion of the rolls, worn roll bearings and/or slightly distorted frame members can cause such measurements to be inaccurate.
Hence, a method for accurately measuring the nip clearance between large rolls rotating close to each other is needed. The measurement must be accurate in spite of thermal expansion of the rolls and other distortions that frustrate prior-art measuring techniques. Nip clearance measurements should be possible even if the rolls are rotating in opposite directions or at different peripheral speeds.